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diff --git a/doc/README.falcon b/doc/README.falcon new file mode 100644 index 0000000..93e855d --- /dev/null +++ b/doc/README.falcon @@ -0,0 +1,209 @@ +U-Boot Falcon Mode +==================== + +Introduction +------------ + +This document provides an overview of how to add support for Falcon Mode +to a board. + +Falcon Mode is introduced to speed up the booting process, allowing +to boot a Linux kernel (or whatever image) without a full blown U-Boot. + +Falcon Mode relies on the SPL framework. In fact, to make booting faster, +U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot +image. In most implementations, SPL is used to start U-Boot when booting from +a mass storage, such as NAND or SD-Card. SPL has now support for other media, +and can generally be seen as a way to start an image performing the minimum +required initialization. SPL mainly initializes the RAM controller, and then +copies U-Boot image into the memory. + +The Falcon Mode extends this way allowing to start the Linux kernel directly +from SPL. A new command is added to U-Boot to prepare the parameters that SPL +must pass to the kernel, using ATAGS or Device Tree. + +In normal mode, these parameters are generated each time before +loading the kernel, passing to Linux the address in memory where +the parameters can be read. +With Falcon Mode, this snapshot can be saved into persistent storage and SPL is +informed to load it before running the kernel. + +To boot the kernel, these steps under a Falcon-aware U-Boot are required: + +1. Boot the board into U-Boot. +Use the "spl export" command to generate the kernel parameters area or the DT. +U-Boot runs as when it boots the kernel, but stops before passing the control +to the kernel. + +2. Save the prepared snapshot into persistent media. +The address where to save it must be configured into board configuration +file (CONFIG_CMD_SPL_NAND_OFS for NAND). + +3. Boot the board into Falcon Mode. SPL will load the kernel and copy +the parameters which are saved in the persistent area to the required address. + +It is required to implement a custom mechanism to select if SPL loads U-Boot +or another image. + +The value of a GPIO is a simple way to operate the selection, as well as +reading a character from the SPL console if CONFIG_SPL_CONSOLE is set. + +Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells +SPL that U-Boot is not the only available image that SPL is able to start. + +Configuration +---------------------------- +CONFIG_CMD_SPL Enable the "spl export" command. + The command "spl export" is then available in U-Boot + mode +CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be + copied by SPL. + In most cases, it is <start_of_ram> + 0x100 + +CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored + +CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved. + +CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied + +CONFIG_SPL_OS_BOOT Activate Falcon Mode. + +Function that a board must implement +------------------------------------ + +void spl_board_prepare_for_linux(void) : optional + Called from SPL before starting the kernel + +spl_start_uboot() : required + Returns "0" if SPL should start the kernel, "1" if U-Boot + must be started. + + +Using spl command +----------------- + +spl - SPL configuration + +Usage: + +spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ] + +img : "atags" or "fdt" +kernel_addr : kernel is loaded as part of the boot process, but it is not started. + This is the address where a kernel image is stored. +initrd_addr : Address of initial ramdisk + can be set to "-" if fdt_addr without initrd_addr is used +fdt_addr : in case of fdt, the address of the device tree. + +The spl export command does not write to a storage media. The user is +responsible to transfer the gathered information (assembled ATAGS list +or prepared FDT) from temporary storage in RAM into persistant storage +after each run of 'spl export'. Unfortunately the position of temporary +storage can not be predicted nor provided at commandline, it depends +highly on your system setup and your provided data (ATAGS or FDT). +However at the end of an succesful 'spl export' run it will print the +RAM address of temporary storage. +Now the user have to save the generated BLOB from that printed address +to the pre-defined address in persistent storage +(CONFIG_CMD_SPL_NAND_OFS in case of NAND). +The following example shows how to prepare the data for Falcon Mode on +twister board with ATAGS BLOB. + +The "spl export" command is prepared to work with ATAGS and FDT. However, +using FDT is at the moment untested. The ppc port (see a3m071 example +later) prepares the fdt blob with the fdt command instead. + + +Usage on the twister board: +-------------------------------- + +Using mtd names with the following (default) configuration +for mtdparts: + +device nand0 <omap2-nand.0>, # parts = 9 + #: name size offset mask_flags + 0: MLO 0x00080000 0x00000000 0 + 1: u-boot 0x00100000 0x00080000 0 + 2: env1 0x00040000 0x00180000 0 + 3: env2 0x00040000 0x001c0000 0 + 4: kernel 0x00600000 0x00200000 0 + 5: bootparms 0x00040000 0x00800000 0 + 6: splashimg 0x00200000 0x00840000 0 + 7: mini 0x02800000 0x00a40000 0 + 8: rootfs 0x1cdc0000 0x03240000 0 + + +twister => nand read 82000000 kernel + +NAND read: device 0 offset 0x200000, size 0x600000 + 6291456 bytes read: OK + +Now the kernel is in RAM at address 0x82000000 + +twister => spl export atags 0x82000000 +## Booting kernel from Legacy Image at 82000000 ... + Image Name: Linux-3.5.0-rc4-14089-gda0b7f4 + Image Type: ARM Linux Kernel Image (uncompressed) + Data Size: 3654808 Bytes = 3.5 MiB + Load Address: 80008000 + Entry Point: 80008000 + Verifying Checksum ... OK + Loading Kernel Image ... OK +OK +cmdline subcommand not supported +bdt subcommand not supported +Argument image is now in RAM at: 0x80000100 + +The result can be checked at address 0x80000100: + +twister => md 0x80000100 +80000100: 00000005 54410001 00000000 00000000 ......AT........ +80000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot +80000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs + +The parameters generated with this step can be saved into NAND at the offset +0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS) + +nand erase.part bootparms +nand write 0x80000100 bootparms 0x4000 + +Now the parameters are stored into the NAND flash at the address +CONFIG_CMD_SPL_NAND_OFS (=0x800000). + +Next time, the board can be started into Falcon Mode moving the +setting the gpio (on twister gpio 55 is used) to kernel mode. + +The kernel is loaded directly by the SPL without passing through U-Boot. + +Example with FDT: a3m071 board +------------------------------- + +To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get +prepard/patched first. U-Boot usually inserts some dynamic values into +the DT binary (blob), e.g. autodetected memory size, MAC addresses, +clocks speeds etc. To generate this patched DT blob, you can use +the following command: + +1. Load fdt blob to SDRAM: +=> tftp 1800000 a3m071/a3m071.dtb + +2. Set bootargs as desired for Linux booting (e.g. flash_mtd): +=> run mtdargs addip2 addtty + +3. Use "fdt" commands to patch the DT blob: +=> fdt addr 1800000 +=> fdt boardsetup +=> fdt chosen + +4. Display patched DT blob (optional): +=> fdt print + +5. Save fdt to NOR flash: +=> erase fc060000 fc07ffff +=> cp.b 1800000 fc060000 10000 +... + + +Falcon Mode was presented at the RMLL 2012. Slides are available at: + +http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf |